# The Naked Scientists Forum

### Author Topic: A Question of thrust in space...  (Read 4109 times)

#### Garret

• First timers
• Posts: 6
##### A Question of thrust in space...
« on: 14/10/2014 18:25:09 »
Hi guys,

You boys look clever, maybe someone could help me with a dilemma concerning the thrust of an object in space. It's for a game I'm designing and I'm doing some research.

I need you to think fighter combat in space. Things will work differently there I know, but I need to get as close to understanding the physics and phenomenon as possible.

If I have a heavier craft than the other and I put them right beside each other, let them thrust away at the same time with no gravitational interference at all, how would they react with the same amount of thrust behind them? will they keep up with each other or do mass still have an effect on the thrust to weight ratio?

Thanks, you've been a great audience, and I appreciate all the help anyone is willing to give me. It's hard work being an average joe ;

Regards

-Garret

#### chiralSPO

• Global Moderator
• Neilep Level Member
• Posts: 1875
• Thanked: 143 times
##### Re: A Question of thrust in space...
« Reply #1 on: 14/10/2014 18:28:06 »
Yes, heavier things require more thrust to change speed than lighter things, even without considering gravity.

The acceleration of the object is equal to the applied force divided by its mass (A = F/m).

#### Garret

• First timers
• Posts: 6
##### Re: A Question of thrust in space...
« Reply #2 on: 14/10/2014 18:56:15 »
Thank you,

I've been wondering about this one for a while. To continue on that question, would something be able to reach a max / terminal velocity?

#### chiralSPO

• Global Moderator
• Neilep Level Member
• Posts: 1875
• Thanked: 143 times
##### Re: A Question of thrust in space...
« Reply #3 on: 14/10/2014 19:11:36 »
There isn't really such a thing as terminal velocity in space because there isn't much source of drag (it turns out space is really close to empty). You can't go any faster than the speed of light (about 300000000 meters per second), but it would be difficult to get going fast enough that this sort of limit would have any bearing. You also have to be careful (particularly when talking about moving around in space) that all motion is relative. Everything is already moving (some of it very quickly), so one must specify what the speed is relative to.

#### Garret

• First timers
• Posts: 6
##### Re: A Question of thrust in space...
« Reply #4 on: 14/10/2014 20:12:31 »
Interesting,

If you say all motion is relative and it should be relative to something, what exactly do you mean by that statement?

Should I compare the movement to something else in it's vicinity or to another "measuring rod"?

Everything moves, from the smallest particle to the biggest planet. The problem I am facing is that our Aerial combat has only been in a atmospheric environment, that means your fighters with a bigger thrust to weight ratio try to out maneuver in a vertical plane instead of a horizontal plane with an opponent who's turn rate will out match it. In space... there's no such thing as going vertical except if you have a gravitational pull in the area. The big thing is, IF... you were to have fuel source that cannot be depleted, you would not have a problem. But if you have a fossil fuel, you need bigger craft for longer range and interception purposes.

But something bulky normally have a poor turn rate in comparison to a more nimble aircraft. I need to figure out how a bigger craft can shake something on it's tale if it can't go vertical. This is something you might have a science point of view on?

Normally your craft with the better turn rate will try to move into a horizontal scissors with his opponent in order to get the area of tail down for a snapshot, but the heavier craft will go vertical or attempt a rolling scissors depending on plane specs. The idea is to break speed without taking down the power while keeping the aircraft maneuverable.

In space.... there's no such thing. Just plane specs. Only place this will work is near a planetary body I presume.

And again. thanks for your input.

Regards

#### chiralSPO

• Global Moderator
• Neilep Level Member
• Posts: 1875
• Thanked: 143 times
##### Re: A Question of thrust in space...
« Reply #5 on: 14/10/2014 21:50:49 »
Well, although vertical and horizontal don't have the same connotations in space as they do on the surface of a planet (or close to it), there are still 3 spacial dimensions to move in. The more massive spacecraft will require proportionally more thrust to maneuver, but if they have more powerful thrusters than the smaller spacecraft, this may not be as big an issue as you suppose. Some of these maneuvers you are talking about could happen around any sufficiently large celestial body (star, planet, moon...), but they would have to be pretty close for there to be a big effect.

#### Garret

• First timers
• Posts: 6
##### Re: A Question of thrust in space...
« Reply #6 on: 14/10/2014 22:19:45 »
I figured as much as it comes to the planetary bodies, but in dead space, you need some way to either bug out or get a way to move in on his six. This off course, is in our stupid thinking that a fight would work the same out there (which I can guarantee it won't) but for the sake of my little hobby and making it fun I should give something the mind can relate to...

So with our primitive way of thinking, one thing that would most assuredly change the most dangerous game on earth is the concept that you can still go in one direction while swinging around your axis. Wheeling and gunning at your opponent. Something the Russians are try to build their aircraft around that sort of maneuverability, but in space it's free. That being said, lets keep the scenario in dead space without any gravitational pull for now.

If you have something heavier, think it will be harder to spin around your axis as it is with thrust? I believe there's jets nozzles on the side of our current craft helping with direction control? logic states that the bigger craft will just need more but will cause the same spin rate if enough power's added right?

For what I am trying to do, I need to understand the concept of a vacuum better and so far you've been giving great information for me to process. I appreciate the help.

Regards

#### alancalverd

• Global Moderator
• Neilep Level Member
• Posts: 4707
• Thanked: 153 times
• life is too short to drink instant coffee
##### Re: A Question of thrust in space...
« Reply #7 on: 14/10/2014 23:26:16 »
I think you need to redefine your entire concept of combat for space, and then apply the laws of physics which are a lot simpler in the absence of gravity and aerodynamics.

Aerial combat is generally a matter of attacking and defending a point on the ground, within the limits of fuel capacity and aerodynamic stress (never mind g, you can always replace the human pilot with a remote control system). Your defence capability is limited by the ability to see the enemy and loiter over his presumed target or probable track.

In the absence of a horizon, cloud, atmospherics, birds, surface vehicles, friendly aircraft, neutrals, and terrain, you can spot a moving attacker in space a long, long way off, and in the absence of gravitation or drag, you can loiter anywhere for as long as you like. So what is his attack trajectory likely to be? "Straight in" would be futile unless he has overwhelming armament and can sustain your counterattack indefinitely, so he is going to pass aside until he has drawn you into a position outside his radius to the objective. Assuming you have conventional rocket or bullet projectiles with a finite speed, you enemy will probably approach on a jinking line or a spiral, fly past the objective, and try to coax you into a tailchase. If you follow him, he will spin round to face you and the objective whilst travelling backwards, slow down (so you overshoot) and release his primary weapon as you are going past him. He now accelerates back the way he came, spins round to face you again. and flies backwards to his base, engaging you with his guns etc as he does so.

#### chiralSPO

• Global Moderator
• Neilep Level Member
• Posts: 1875
• Thanked: 143 times
##### Re: A Question of thrust in space...
« Reply #8 on: 15/10/2014 00:26:18 »
Assuming you have conventional rocket or bullet projectiles with a finite speed,

Yes, everything is going to travel at finite speed.

Rocket-propelled munitions would probably be quite useful, but I wouldn't count on bullets being used to fight in deep space. Bullets are probably too slow for the velocities and distances likely involved in deep space combat. I probably wouldn't like the idea of such recoil for the spaceship either, unless it were truly massive.

As trite as it might sound, I think space combat would probably be best done with directed EM or accelerated particle weapons. Sufficiently powerful lasers or synchrotrons could deliver bursts of EM radiation (infrared all the way down to sub Å x-rays) that could be selected depending on the material of the target. Particle accelerators could also hurl electrons or protons or ionized matter at the target at relativistic speeds.

Whatever weaponry is employed will probably be limited by how heavy the "gun" is and how much ammo (energy included; ie propellant, payload, batteries, fuel, power source etc.) is brought along. The weapon that inflicts the most damage per mass of stuff on board will probably be what's employed.

#### Garret

• First timers
• Posts: 6
##### Re: A Question of thrust in space...
« Reply #9 on: 15/10/2014 05:59:03 »
I have to agree that this is a completely new way of thinking and will eliminate a tail chase situation completely. The same goes for the bullets and munitions as we know it. Thanks for that.
In our time, 2 opponents rarely go for a guns pass in a head on because of the speed involved, so much more for space then.

I understand that something like this throws over the apple cart. All we have learned about aerial combat will go out the door once something like this becomes a reality. Having a fixed weapon system will not be a good idea since you'll need your angular velocity in relation to the target high on your approach.

It seems what we are saying is, the age of the fighter would die. Am I correct in such a statement?

Concerning the weapon systems, I was actually aiming towards asking about missiles but you already answered that one indirectly. Remote control of the craft... this is the big question of the USA Navy today, do they go unmanned or not. You lose a lot of situational awareness in a drone and you lose the feel of your environment which is very important if things go wrong. Most of them fighter bubba's will tell you it's 90% feeling. And if combat will become even more complex in space, I don't think remote control will hack it. But since this is all theory and new grounds, nothing is impossible yet.

I do not know much about particle acceleration or the concept of concentrated light, but if that's the only eligible weapon system, I guess range would be greatly reduced then? Further more would something like a laser burn through metallic surfaces or would it be impact that causes destruction?

Regards

#### alancalverd

• Global Moderator
• Neilep Level Member
• Posts: 4707
• Thanked: 153 times
• life is too short to drink instant coffee
##### Re: A Question of thrust in space...
« Reply #10 on: 15/10/2014 09:23:22 »
It's a historical fact that every general fights the last war, not the next one. Partly, I suspect, because many of them are history or classics graduates!

So let's look at the aggressor's objective and consider how he is going to attack it, then work out how we can defend it: it may turn out that the fighter is indeed obsolete.

If he uses a directed energy weapon, distance is limited only by the angular spread of the beam. The objective won't see a photon beam coming until it hits us, and charged particles travelling at a constant 0.999c won't be detectable until they are very close indeed. So bombardment is a matter of building a sufficiently large source and aiming it in the right direction, which is fine if the objective is travelling on a predictable course. Thus a planet or even a large space station cannot be defended except by preemptive strike.

The fighter does become relevant if the aggressor wants to invade or capture the objective rather than just damage it. But once he is in close range, we are back to fairly conventional fighter tactics, albeit in vacuo and with negligible gravitation, and rockets, guided or unguided, would seem to be the appropriate ordnance for use against a troop carrier. You can use a proximity warhead with large chunks of shrapnel as there is no terminal speed limit. The trick, therefore, is to get behind the aggressor to reduce the closing speed and give your missiles a chance to track him. If he is using chemical rocket propulsion, an oldfashioned infrared homer will do the trick, though you will need to replace the guidance fins with lateral thrust jets. Range isn't a problem: once you have established a positive closing speed, you only need small bursts of power to change direction, so we are looking at a liquid-fuelled Sidewinder.

#### evan_au

• Neilep Level Member
• Posts: 4113
• Thanked: 245 times
##### Re: A Question of thrust in space...
« Reply #11 on: 15/10/2014 10:19:44 »
A major barrier to operation of drones in space is the speed-of-light lag. Even at the distance of the Moon, a delay of 1 second will make it effectively impossible to hit a target.

So a fair degree of autonomy would be required in a drone. (Skynet, anyone?)

If a planet had few space resources, a dense asteroid painted black may prove hard to detect, and then impossible to deflect in time.

#### Garret

• First timers
• Posts: 6
##### Re: A Question of thrust in space...
« Reply #12 on: 15/10/2014 11:25:14 »
I think if we talk about a weapons platform that can shoot anywhere in a 360 sphere, it will be all about angular velocity and tracking speed as in the space simulator EVE: Online.

But tracking at those speeds in a high deflection angle, against a target thats constantly changing it's direction, no computer in the world will be able to calculate that, because it's no longer calculation but speculation. I think if you start talking about carrier battle groups in space, the fighter might not be absolute depending on the defensive systems that might play a role in counter measures.

That being said, the question will be if basic fighter tactics would still play a role as attacker and defender. If you have a fixed weapon system, getting on the targets six is important, missiles should be near useless to fast moving targets like that in space, since changing direction is much easier. A missile should have trouble tracking anything going beyond a 2 G turn, but that was in the 70's
If a craft could be so maneuverable, I guess a few fixes to a missiles tracking tech should also do the trick.

I think I got my answer to the big question I had and it seems it lies in conventional thrust tech like we know it or gravitational thrust like we would describe what some people might call UFO's and the theories around that. So I presume if I'm to create a universe, will it be in a time of conventional thrust or gravitational?
It seems gravitational thrust will change everything, changing "fighter combat tactics" itself.

Say hello to space, cause Top Gun is going bye bye.

Thank you all for the participation and helping me out with some great ideas, as I said before, I appreciate all the help you guys gave me. You made me think further.

Regards

-Garret, South-Africa

#### alancalverd

• Global Moderator
• Neilep Level Member
• Posts: 4707
• Thanked: 153 times
• life is too short to drink instant coffee
##### Re: A Question of thrust in space...
« Reply #13 on: 15/10/2014 16:31:21 »
Range isn't a problem: once you have established a positive closing speed, you only need small bursts of power to change direction, so we are looking at a liquid-fuelled Sidewinder.

It just occured to me that this is really, really fundamental. Space combat is much more like naval warfare - infinite endurance and battlefield, but nowhere to hide. So the best sources of tactics are likely to be Nelson and Shephard/Lindemann (the pursuit of the Bismarck) rather than Johnson and Immelman.

#### The Naked Scientists Forum

##### Re: A Question of thrust in space...
« Reply #13 on: 15/10/2014 16:31:21 »